At hospitals, test centers, etc. in recent years, there have been attempts for automating various tests including clinical tests and/or saving an amount of measuring time in view of a shortage of manpower, cost-cutting purposes, demands for treating large quantities of samples, etc. For the technique suitable for this automation, a method where antigenic substances are subjected to a qualitative or quantitative assay with insoluble magnetic carrier particles has drawn attention. One of the objects for utilizing such insoluble magnetic carrier particles is to readily carry out an unavoidable B/F separation in the assay with a magnetic field action.
In addition, in the immunochemical field where such insoluble magnetic carrier particles are utilized, there have been used antigen or antibody-bound insoluble magnetic particles (sensitized insoluble magnetic particles) wherein the said magnetic carrier carries an antigen or antibody identical with a target analyte to be assayed. The publicly known ones in the present field are those which (for example, as described in JP, A, 06-160387 (1994) and JP, A, 07-72, 155 (1995)) comprise, in the case of assaying for an antigenic substance in a fluid (test sample) such as a biological sample, the step of mixing the said test sample with insoluble magnetic particles on which an antibody capable of specifically binding to the said antigenic substance or a fragment thereof is pre-adsorbed and carried, and the step of then measuring the degree of the said antigenic substance bound to the said insoluble magnetic particles whereupon the said antigenic substance will be detected or quantitated.
However, when antibodies, etc. are previously adsorbed on insoluble magnetic particles and used for the assay, the stability problem of the sensitized insoluble magnetic particles is unavoidable. Therefore, regardless of every effort, there still remain problems including the best-before-period issue of the reagents, difficulty in preparing the same, etc. Further, there is another problem that, when preserved for a long period, the sensitized insoluble magnetic particles are apt to cause precipitates in a suspension thereof.
In the introduction of test instruments for fully-automated clinical tests, etc. for saving labor or for treating large quantities of samples within a short time, it is usually necessary to conduct washing treatments for the B/F separation in the assay. To reduce even a few steps or carry out the assay in a simple fashion is an unavoidable proposition for making the measurement efficient and rapid. For these purposes, it has been recently attempted to develop an immunoassay system where insoluble magnetic particles are used as carriers but it is unavoidable to lose the insoluble magnetic particles used therefor during the washing step. That is also a cause for consuming precious antibodies carried on said carrier, etc. As such, in the automated clinical test instruments using magnetic particles, there is a disadvantage that more antibodies, etc. are required than in the case of usual immunoassay because of the use of relatively large quantities of magnetic particles. In addition, there is a problem in the particle loss due to washing for many times.